Peroxide-Free and Amine-Free Redox Free Radical Polymerization: Metal Acetylacetonates/Stable Carbonyl Compounds for Highly Efficient Synthesis of Composites

Abstract

New peroxide-free, amine-free, and phosphine-free redox free radical polymerization (RFRP) initiating systems comprising remarkably stable (i) metal acetylacetonates (Mn(acac)3, Cu(acac)2) and (ii) carbonyl compounds bearing labile hydrogen in the α-position are presented for polymerization initiation under mild conditions (under air, at room temperature, nonpurified monomers). The systems proposed in this work are competitive or even outranked the well-known peroxide-based RFRP reference in several criteria: (i) toxicity, (ii) stability, (iii) surface curing, (iv) overall double-bond conversions, and (v) workability of the RFRP mixture (longer gel times are now possible). Radical initiating reactions are studied using many complementary experimental/theoretical techniques: optical pyrometry, thermal imaging, Raman confocal microscopy, electron spin resonance (ESR), ESR spin trapping (ESR-ST), high-resolution electrospray ionization mass spectrometry (HR-ESI-MS), density functional theory (DFT), simulations of bond dissociation energies (BDE), reaction enthalpies, and DFT simulations of seven unknown ESR-ST adducts. A full consistent picture of the chemical mechanisms involved in these new redox systems is provided.